The classical X-ray sources that have been used for sterilization are generally either radioactive nuclides or high voltage vacuum tubes, both of which have limited applications. The radioactive nuclides typically have comparatively short lives and are expensive and dangerous to handle. In addition, the gamma or X-ray intensity they produce cannot be readily monitored and, once activated, the sources cannot be turned off. The high voltage vacuum tube apparatus requires heavy and cumbersome high voltage supplies, which are also dangerous to handle. Further, this apparatus needs highly qualified personnel to operate and maintain it.
To avoid these deficiencies it was recently proposed to use a compact cyclic electron accelerator as a source of X-ray in the range of 150 keV photon energy. See H. R. Gardner, T. Ohkawa, A. M. Howald, A. W. Leonard, L. S. Peranich and J. R. D'Aoust, Rev. Sci. Instruments, 61(2), February 1990, pages 724-727. In this source, the electrons are injected from an electron emitter into a vacuumated resonant cavity, and accelerated by a microwave field along a spiral orbit to a molybdenum target, which produces the X-ray radiation. Although this source advantageously avoids the use of radioactive nuclides and a high voltage supply, it is not a reliable tool for routine use in the industry and agriculture. The reasons are that the emitted X-ray intensity is too weak, and the life of the device is limited by the life of an electron emitter, which cannot readily be replaced without the replacing of the entire vacuumated cavity. Also, since the X-rays are produced by an electron beam striking a molybdenum target, only half of the produced X-rays are reflected from the target, to be useful, while the other half are absorbed in the target and lost. Further, this source cannot produce homogeneous X-ray radiation in all radial directions from a central source, such as is useful, for example, in sterilizing fluids, e.g., water, from a source centrally located within the flow of the fluid as described below.
There remains a continuing need for better sources of X-rays, and in particular for economical X-ray sources having sufficient intensity for surface and volume sterilization of goods and food stuffs.